Final answer:
Melanocyte-stimulating hormone (MSH) increases skin pigmentation when it is present in excess, leading to conditions such as hyperpigmentation or darker skin during pregnancy.
Step-by-step explanation:
The hormone that increases skin pigmentation when present in excess is melanocyte-stimulating hormone (MSH). MSH is responsible for stimulating melanin production, leading to darkening of the skin. Conditions like Addison's disease or tumors of the pituitary gland can cause increased secretion of MSH, which results in hyperpigmentation. Additionally, during pregnancy, increased MSH levels, in conjunction with estrogens, can lead to darker pigmentation of the areolas and the development of a linea nigra, a line of pigment from the umbilicus to the pubis.